Catheter with tapered compliant balloon and tapered stent

ABSTRACT

A balloon comprising: a center portion having a proximal end, a distal end opposite the proximal end, and a length between the proximal end and the distal end. The center portion comprises: a first nominal diameter and a first radial modulus at the proximal end; a second nominal diameter and a second radial modulus at the distal end; further wherein, the first nominal diameter is equal to the second nominal diameter, such that, when the balloon is inflated to a nominal pressure, the center portion has a constant diameter over the length; and further wherein, the first radial modulus is smaller than the second radial modulus, such that, when the balloon is inflated above a nominal pressure, the center portion adopts a tapered shape in which the proximal end has a first stretched diameter and the distal end has a second stretched diameter, the first stretched diameter being larger than the second stretched diameter.

BACKGROUND

This application relates to balloon catheters for medical purposes suchas angioplasty and stent delivery, and to stents suitable for deliverywith such catheters.

Balloon catheters are well known in the art. Balloon catheters have beendeveloped for various purposes including angioplasty, stent delivery,and many other applications in which medical devices must be expandedwithin a body cavity of a patient. The balloon is inserted inside themedical device on the tip of a catheter, and when the device has beensuccessfully introduced into a body cavity via the vasculature of thepatient, the balloon is expanded by a fluid medium transmitted via alumen in the catheter. The expanding balloon expands the device by anamount that can be adjusted by the operating physician throughvisualization means such as fluoroscopy. FIG. 1 shows a known cathetersystem 10, which is tipped by a balloon 16 at its distal end upon whicha stent 12 is mounted for delivery. A sheath 18 may cover thestent/balloon combination during delivery, and may be withdrawn prior todeployment of the stent 12.

Balloons on catheters have been provided with various properties. Someballoons have been configured to be compliant, which is to say, elastic.Under such structure, the greater the internal pressure, the greater thediameter of the expanded balloon. Some balloons have been configured tobe non-compliant, which is to say inelastic. Such inelastic balloonshave substantially only one expanded diameter, so that the operatingphysician can be assured that when the device is implanted, it willassume only one final diameter under a range of pressures.

However, in some procedures, it is desired by a physician for a balloonto assume a non-uniform diameter. Such a situation may arise where astent having a substantial length is to be implanted in a vessel. Whilemost vessels show no appreciable taper over a short length such as over20 mm-30 mm, it is common for a vessel to taper appreciably over asubstantial length, such as for example from 40 mm-80 mm. Specifically,the coronary artery lumen has unique character. It is well-known thatthe left anterior descending (LAD) artery diameter is typically notconstant, and that it typically tapers narrower in its distal course.This is in comparison with the right coronary artery (RCA) which is morecylindrical over its course. It is estimated that the LAD loses 15% ofits diameter for every 30 mm in its length.

FIGS. 1A-1C show a catheter with a balloon/stent mounted at a distaltip, in a body lumen 14 of a patient that tapers narrower along itsdistal course. In cases where a physician wishes to implant a stenthaving substantial length, the physician may be confronted by one orboth of two problems. First, the physician may initially conclude thathe/she is confronted with an artery that is substantially tubular with aconstant diameter over the length. Therefore, he/she may select aballoon/stent capable of achieving a substantially tubular shape overthe length. However, as she proceeds to deploy the stent, it may becomeapparent that the artery does in fact possess a taper along its length.Where, as in the case of a left anterior descending artery, the taper is15% it will be appreciated that the cone angle of taper is 8 degrees. Itis therefore quite possible that the surgeon discovers during deploymentthat the artery is in fact substantially tapered.

The problem that then may arise is that, having chosen a balloon/stentcombination having a constant diameter, it may transpire that it issized correctly at the proximal end in an expanded condition, but is toolarge at the distal end. Alternatively, he/she may be left with aconstant diameter balloon/stent that is correctly sized at the distalend but is too small at the proximal end. This latter condition isexemplified in FIGS. 2 and 3. Thus, the physician, hoping to implant asubstantially long stent that is uniformly shaped along its length andplaced on a balloon having a uniform expanded shape along its length,may be compelled to compromise, and size the final expanded diameter tofit the vessel in the middle of the stent, and have a proximal end thatis too small and a distal end that is too large for the vessel.

It is known how to impart an actual taper to a balloon. Typically, suchis accomplished by imparting an initially tapered shape at the moldingstage, and then upon inflation by expansion medium, the balloon adopts atapered shape at nominal pressure, and continues to possess a taperedshape throughout the inflation process. This type of balloon howeverdoes not give the physician a choice of inflating the balloon to auniform diameter at nominal pressure in the event that a uniform vesselis encountered, and then, in the event that it turns out that the vesselis tapered, to continue to inflate the balloon beyond nominal pressureto increase the diameter of the balloon only at the proximal end, whileleaving the diameter of the balloon substantially unchanged at thedistal end—thereby producing a suitable tapered balloon.

Thus there is a need in the art for a balloon that expands to a constantdiameter at nominal pressure, but which expands to a tapered diameter inexcess of nominal pressure. The present invention addresses these andother needs.

A corollary need in the art is for a stent that is suitable for use inconjunction with any of the balloons described herein.

The present invention addresses these, and other needs.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a balloon for attachment to a distalportion of a medical catheter. The balloon comprises a center portionhaving a proximal end, a distal end opposite the proximal end, and alength between the proximal end and the distal end. The center portionfurther comprises a first nominal diameter and a first radial modulus atthe proximal end; and a second nominal diameter and a second radialmodulus at the distal end. The first nominal diameter is equal to thesecond nominal diameter, such that, when the balloon is inflated to anominal pressure, the center portion has a constant diameter over thelength. Furthermore, the first radial modulus is smaller than the secondradial modulus, such that, when the balloon is inflated above a nominalpressure, the center portion adopts a tapered shape in which theproximal end has a first stretched diameter and the distal end has asecond stretched diameter, the first stretched diameter being largerthan the second stretched diameter.

In some embodiments, the center portion comprises a compliant polymermembrane which has a first thickness at the proximal end and a secondthickness at the distal end, wherein the first thickness is less thanthe second thickness.

In other embodiments, the center portion comprises a compliant polymermembrane.

A plurality of successive threads are wrapped circumferentially aroundthe center portion to reinforce the center portion, the threads beingspaced along the center portion at a constant pitch and being adhesivelyattached to the center portion. Further, an initial successive thread islocated at the proximal end and has a first cross sectional area. Afinal successive thread is located at the distal end and has a secondcross sectional area. A medial successive thread is located between theinitial successive thread wherein the final successive thread has athird cross sectional area. Further, the first cross sectional area issmaller than the second cross sectional area and the third crosssectional area is larger than the first cross sectional area but smallerthan the second cross sectional area. In further embodiments, the centerportion comprises a compliant polymer membrane. An initial twoconsecutive threads are located at the proximal end and have a firstpitch between them; a final two consecutive threads are located at thedistal end and have a second pitch between them; and a medialconsecutive two threads are located between the initial two consecutivethreads and the final two consecutive threads and have a third pitchbetween them, wherein the first pitch is larger than the second pitchand the third pitch is smaller than the first pitch but smaller than thethird pitch. In yet further embodiments, the center portion comprises acompliant polymer membrane. A first thread is wound in a helix along thecenter portion and a first two successive windings are located at theproximal end and have a first pitch. A final two successive windings arelocated at the distal end and have a second pitch, and a medial twosuccessive windings are located between the first two successivewindings and the final two successive windings and have a third pitch.The first pitch is larger than the second pitch and the third pitch issmaller than the first pitch but larger than the third pitch.

In yet a further embodiment, the center portion comprises a compliantpolymer membrane. An initial successive thread is located at theproximal end and is formed from a material having a first elasticmodulus. A final successive thread is located at the distal end and isformed from a material having a second elastic modulus. A medialsuccessive thread is located between the initial successive thread andfinal successive thread, and is formed from a material having a thirdelastic modulus. The first elastic modulus is smaller than the secondelastic modulus and the third elastic modulus is larger than the firstelastic modulus but smaller than the second elastic modulus.

In another embodiment, the invention is a stent for insertion into avessel of a patient. The stent comprises a plurality of rings that aresuccessively connected to each other by a plurality of links, theplurality of rings extending in an axial direction from a first ring ata proximal end followed by a plurality of succeeding rings to a finalring at a distal end. Each succeeding ring is preceded by a precedingring. Each of the plurality of rings includes a plurality of adjacentpeaks and valleys, wherein each valley is connected to an adjacent peakby a strut to provide an undulating pattern within each ring. Each ofthe plurality of rings has a compressed condition for delivery into thepatient and an expanded condition after deployment in the patient,wherein, in the compressed condition each preceding ring has a precedingring length measured in the axial direction and each succeeding ring hasa succeeding ring length measured in the axial direction, wherein aratio of each succeeding ring length divided by each preceding ringlength is a constant number that is smaller than unity.

In some embodiments, the first ring has a first ring length and isconnected to a second ring by a first link having a first link length,the first link length being equal to the first ring length. In furtherembodiment, the ratio is in a range of 0.90 to 0.95.

In yet a further embodiment, the invention is a method of expanding astent within a vasculature of a patient. The method comprises disposinga stent upon a balloon that is deflated, the balloon comprising a centerportion having a proximal end, a distal end opposite the proximal end,and a length between the proximal end and the distal end; inserting theballoon inside the vasculature of the patient; inflating the balloon toa nominal pressure and, simultaneously, imparting a cylindrical shape tothe center portion of the balloon; and further inflating the balloon toa pressure beyond nominal pressure and, simultaneously, imparting atapered shape to the center portion of the balloon. In some embodiments,imparting a cylindrical shape to the center portion of the balloonincludes imparting a cylindrical shape to the stent. In furtherembodiments, imparting a tapered shape to the center portion of theballoon includes imparting a tapered shape to the stent.

These and other advantages of the invention will appear when read inconjunction with the description of the drawings and detaileddescription of some embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a delivery catheter that is known in theart, tipped by a balloon at a distal end, upon which is mounted a stent.

FIG. 1B is a schematic view of a distal tip of the catheter of FIG. 1A,in the process of expanding the balloon for deployment in a vessel thattapers down in the distal direction.

FIG. 1C is a schematic view of a stent having a constant diameter knownin the art that has been deployed in a vessel of a patient.

FIG. 2 is a schematic side elevational view of a first embodiment of aballoon having features of the invention.

FIG. 3 is a sectional view of the embodiment of FIG. 2.

FIG. 4 is a schematic side elevational view of a second embodiment of aballoon having features of the invention.

FIG. 5 is a sectional view of the embodiment of FIG. 4.

FIG. 6 is a schematic side elevational view of a third embodiment of aballoon having features of the invention.

FIG. 7 is a sectional view of the embodiment of FIG. 6.

FIG. 8 is a schematic side elevational view of a fourth embodiment of aballoon having features of the invention.

FIG. 9 is a sectional view of the embodiment of FIG. 8.

FIG. 10 is a schematic side elevational view of a fifth embodiment of aballoon having features of the invention.

FIG. 11 is a sectional view of the embodiment of FIG. 10.

FIG. 12 is a “rollout” view of a stent, in unexpanded condition,suitable for use with any embodiment of the balloons in FIGS. 2-11.

FIG. 13 is a “rollout” view of the stent shown in FIG. 12, in expandedcondition.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Referring now to the drawings, wherein like reference numbers are usedherein to designate like elements throughout, the various views andembodiments of a balloon configured for delivering a tapered stent areillustrated and described, and other possible embodiments are described.The figures are not necessarily drawn to scale, and in some instancesthe drawings have been exaggerated and/or simplified in places forillustrative purposes only. One of ordinary skill in the art willappreciate the many possible applications and variations based on thefollowing examples of possible embodiments.

FIGS. 2-3 exemplify a first embodiment of the invention. FIG. 2 is aside sectional view of a balloon 50 configured for being fixed to thedistal tip of a known catheter such as shown in FIG. 1A. The balloon 50may be formed of a compliant membrane 51 formed of a suitable polymermaterial. A proximal section 52 of the balloon is configured to have anoutwardly extending conical shape extending from the center towards theouter diameter of the balloon in the distal direction, and a distalsection 54 configured to have an inwardly extending conical shapeextending from the outer diameter of the balloon towards the center ofthe balloon in the distal direction. A central section 56, having aproximal end 60 and a distal end 62, joins the proximal section 52 tothe distal section 54. The thickness of the membrane 51 is structured tohave a thickness T1 at a first point where the proximal section 52 joinsthe central section 56, and a thickness T2 at a second point where thecentral section 56 joins the distal section 54. Importantly in thisembodiment, T1 is thinner than T2. The thickness between the first pointand the second point varies linearly between T1 and T2. (The membranemay be given this linearly varying thickness during the molding of theballoon by using an outer shaping mandrel with cylindrical internal boresurface, and an inner shaping mandrel with conical shaped exteriorsurface.) It will be understood that this configuration gives theballoon a higher radial modulus at the distal end, and a lower radialmodulus at the proximal end. The term “radial modulus” means herein theamount of force that is needed to stretch the balloon membrane a certainamount in a radial direction. Because the pressure inside the balloon isthe same at every point inside the balloon, the force applied inside theballoon, per unit length of the balloon, is the same at every pointalong the length of the balloon. Therefore, at a constant pressure whenthe balloon membrane is being stretched, the portion of the balloonhaving a higher radial modulus will stretch less than the portion of theballoon having the lower radial modulus. Accordingly, the higher radialmodulus at the distant end of the balloon will cause the balloon at thedistal end to stretch, radially, less than the balloon will stretch atthe proximal end where the radial modulus is lower relative to thedistal end.

The native balloon in this embodiment is initially formed to possess aconstant uniform outer diameter over the central section 56 (as shown inFIG. 2) over its length at nominal pressure. (“Nominal pressure” is usedherein to mean a pressure in the balloon that is sufficient to expandthe balloon to remove all folds and wrinkles in the membrane 51 of theballoon, but not sufficient to place the membrane of the balloon under atensile stress which is to say, it will not “stretch” the balloonmembrane. The term “nominal diameter” of a balloon is used herein tomean a diameter that is achieved under nominal pressure.)

The result of this structural arrangement of the varying thickness ofthe balloon 50 may be understood with reference to FIGS. 2-3. Upondelivery of the balloon to the desired location within the vascularanatomy, the deflated balloon may be inflated to nominal pressure, whichwill cause the central section 56 of the balloon to achieve its constantdiameter cylindrical shape along the length of the central section 56,as shown in FIG. 2, under which pressure the balloon membrane merelyunfolds but does not begin to stretch. At this stage, the physician mayassess, using known visualization techniques such as fluoroscopy,whether a satisfactory degree of apposition between the stent (not shownin FIG. 2) mounted on the balloon 50 and the vessel wall has beenachieved in circumstances where the vessel may taper downwardsubstantially towards the distal end. If the visualization shows thatthe taper of the vessel has left insufficient apposition at the proximalend, the physician may elect to continue to inflate the balloon to ahigher pressure than nominal—under which circumstances the balloonmembrane will begin to stretch. The result of such further inflation maybe visualized by reference to FIG. 3, which shows the balloon 50expanded to a larger diameter on the proximal end 60 than on the distalend 62, thereby imparting a distinct taper to the balloon over itslength.

By relying on the emergence of this taper after nominal pressure hasbeen reached and further inflation of the balloon is applied, thephysician may elect to continue to inflate the balloon higher thannominal pressure, thereby improving the apposition of the stent over itslength because it has been given a tapered profile to match the taper ofthe vessel.

FIGS. 4-5 exemplify a second embodiment of the invention. FIG. 4 is aside elevational view, in partial cutaway section, of a balloon 100configured for being fixed to the distal tip of a known catheter such asshown in FIG. 1. The balloon 100 may be formed of a compliant orsemi-compliant membrane 101 formed of a suitable polymer material. Aproximal section 102 of the balloon is configured to have an outwardlyextending conical shape extending from the center towards the outerdiameter of the balloon in the distal direction, and a distal section104 configured to have a inwardly extending conical shape extending fromthe outer diameter of the balloon towards the center of the balloon inthe distal direction. A central section 106, having a proximal end 111and a distal end 112, joins the proximal section 102 to the distalsection 104. The native balloon in this embodiment is initially formedto possess a constant uniform diameter over the central section 106 (asshown in FIG. 4) over its length at nominal pressure, and also aconstant thickness of the membrane 101.

Further shown schematically are circumferential threads 108 that runcircumferentially around the outside of the balloon to reinforce theballoon and add a controllable response to internal balloon pressure aswill be described herein. These threads may either be annular in shape,and slipped over the balloon at a constant pitch P1 when the balloon isnominally inflated, or it may be wound around the balloon in a helicalspiral having a constant pitch, when the balloon is nominally inflated.In either case, threads 108 may be attached to the surface of theballoon using a liquid adhesive, in known manner. In some embodiments,axial threads 110 may be applied and adhered to the outside of theballoon to extend horizontally, in order to reinforce and limit theexpansion of the balloon along its longitudinal axis under inflation.

In the embodiment being described in FIGS. 4-5, the circumferentialthreads 108 may be selected to be compliant or semi compliant. Toaccomplish this objective, the threads may be made from a suitablepolymer. In this embodiment, the circumferential threads are configuredto have a diameter that increases as the threads move from the proximalend of the balloon (here, the left end of the balloon) to the distal endof the balloon (here, the right end). This effect may be envisaged byreference to FIGS. 4-5 where a schematic representation of graduallythickening threads 108 is shown. Where the threads are annuli affixed tothe external surface of the balloon, each annulus may be formed topossess a slightly larger diameter than those adjacent, in anincrementally increasing fashion. Where the threads 108 comprise asingle thread helically wound about the balloon, the diameter of thesingle thread increases from the proximal end to the distal end givingrise to the same effect, namely that the balloon is more rigidlyconstrained against radial expansion at the distal end, and less rigidlyconstrained against expansion at the proximal end. It will be understoodthat this configuration gives the balloon a higher radial modulus onaverage at the distal end, and a lower radial modulus on average at theproximal end.

The result of this structural arrangement of the threads 108 around theballoon may be understood with reference to FIGS. 4-5. Upon delivery ofthe balloon to the desired location within the vascular anatomy, thedeflated balloon may be inflated to nominal pressure, which will causethe balloon to achieve its constant diameter cylindrical shape along itslength, as shown in FIG. 4. At this stage, the physician may assess,using known visualization techniques such as fluoroscopy, whether asatisfactory degree of apposition between the stent (not shown in FIG.4) and the vessel wall has been achieved in circumstances where thevessel tapers downward substantially towards the distal end. If thevisualization shows that the taper of the vessel has left insufficientapposition at the proximal end, the physician may elect to continue toinflate the balloon to a higher pressure. The result of such furtherinflation may be visualized by reference to FIG. 5, which shows theballoon 100 expanded to a larger diameter on the proximal end 102 thanon the distal end 104, thereby imparting a distinct taper to the balloonover its length. By relying on the emergence of this taper after nominalpressure has been reached and further inflation of the balloon isapplied, the physician may elect to continue to inflate the balloonhigher than nominal pressure, thereby improving the apposition of thestent over its length.

FIGS. 6-7 exemplify yet another embodiment of the invention, a balloon200. The balloon 200 may be formed of a compliant membrane 201. Aproximal section 202 of the balloon is configured to have an outwardlyextending conical shape extending from the center towards the outerdiameter of the balloon in the distal direction, and a distal section204 configured to have an inwardly extending conical shape extendingfrom the outer diameter of the balloon towards the center of the balloonin the distal direction. A central section 206, having a proximal end211 and a distal end 212, joins the proximal section 202 to the distalsection 204.

The underlying balloon membrane 201 here is the same as the membrane 101in the previous embodiment. However, in this embodiment, the threads 208around the balloon membrane have a constant diameter throughout thelength of the balloon. Further, the threads 208 are in the form ofannuli that are slipped onto and adhered to the external surface of themembrane at nominal pressure. In this embodiment, however, the pitch ofthe annuli does not remain constant. Rather, the pitch of the annulistart at a set pitch P2 on the proximal end of the balloon, the pitchgradually decreasing to a smaller pitch P4 at the distal end of theballoon via an intermediate size pitch P3 in the middle. This gives riseto the effect that the balloon is more rigidly constrained againstradial expansion at the distal end, and less rigidly constrained againstexpansion at the proximal end. In other words, this configuration givesthe balloon a higher radial modulus on average at the distal end, and alower radial modulus on average at the proximal end. The advantage ofthis arrangement has been described and explained above with respect tothe embodiment in FIGS. 4-5, and is no less advantageous.

FIGS. 8-9 exemplify yet another embodiment of the invention, balloon300. The balloon 300 may also be formed of a compliant membrane 301. Aproximal section 302 of the balloon is configured to have an outwardlyextending conical shape extending from the center towards the outerdiameter of the balloon in the distal direction, and a distal section304 configured to have an inwardly extending conical shape extendingfrom the outer diameter of the balloon towards the center of the balloonin the distal direction. A central section 306, having a proximal end311 and a distal end 312, joins the proximal section 302 to the distalsection 304.

The underlying balloon membrane 301 here is the same as the membrane 101above. However, in this embodiment, the threads 308 around the balloonmembrane have a constant diameter throughout the length of the balloon.Further, the threads 308 are in the form of a single thread wound aroundthe exterior of the membrane 301 at nominal pressure. In thisembodiment, however, the pitch of the single wound threads 308 has ahelical pitch that starts at a set pitch P5 on the proximal end of theballoon, the pitch gradually decreasing to a smaller pitch P7 at thedistal end of the balloon with an intermediate pitch P6 in the middle.It will be appreciated that, as the pitch moves to a smaller amount, theangle of each thread changes from a shallow angle A to a steep angle B.This gives rise to the effect that the balloon is more rigidlyconstrained against radial expansion at the distal end, and less rigidlyconstrained against expansion at the proximal end. In other words, thisconfiguration gives the balloon a higher radial modulus on average atthe distal end, and a lower radial modulus on average at the proximalend.

The advantage of this arrangement has been described and explained abovewith respect to the embodiment in FIGS. 4-5, and is no lessadvantageous.

FIGS. 10-11 exemplify yet another embodiment of the invention, balloon400. The underlying balloon membrane 401 here is the same as themembrane 101 above. The balloon 400 may also be formed of a compliantmembrane 401. A proximal section 402 of the balloon is configured tohave an outwardly extending conical shape extending from the centertowards the outer diameter of the balloon in the distal direction, and adistal section 404 configured to have an inwardly extending conicalshape extending from the outer diameter of the balloon towards thecenter of the balloon in the distal direction. A central section 406,having a proximal end 411 and a distal end 412, joins the proximalsection 402 to the distal section 404.

However, in this embodiment, the threads 408 around the balloon membranehave been selected to possess a constantly changing elastic modulus. Inthis embodiment, the pitch of the annuli may remain constant at a pitchof P8. Under this embodiment, the center portion 406 of the balloon isdivided into sub zones, for example proximal zone 406 a, center zone 406b, and distal zone 406 c. It will be appreciated that three zones areexemplary, and that more than, or fewer than, three zones may be used.In the proximal zone 406 a, the threads 408 are selected for having ahighly compliant modulus of elasticity, and may be made from a suitablepolymer. In the center zone 406 b, the threads are selected for having asemi-compliant modulus of elasticity, and may be made from a suitablepolymer. In the distal zone 406 c, the threads are selected for having anon-compliant modulus of elasticity, and may be made from a suitablepolymer. It will be appreciated that, if the designer wishes to achievea smoother transition of elasticities along the length of the balloon,then the threads may be made a mixture of the identified materials, witha stronger admixture of non-compliant material being added as thethreads are added towards the distal end. This structural arrangementgives rise to the effect that the balloon 403 is more rigidlyconstrained against radial expansion at the distal end, and less rigidlyconstrained against expansion at the proximal end. In other words, thisconfiguration gives the balloon a higher radial modulus on average atthe distal end, and a lower radial modulus on average at the proximalend.

The advantage of this arrangement has been described and explained abovewith respect to the embodiment in FIGS. 4-5, and is no lessadvantageous.

Thus, a number of balloon embodiments are described that produce aballoon that adopts a constant cylindrical diameter at nominal pressure,but that adopts a tapering form at pressures above nominal.

Turning now to a stent configuration that is highly appropriate for usein combination with the balloon embodiments that have been described,the stent configuration is described with reference to FIGS. 12 and 13.

FIG. 12 is a “rollout view” of a stent 500 having features of anembodiment of the invention in a condition before it is deployed(compressed or crimped condition) and FIG. 13 shows the same stent in arollout condition after it is deployed (expanded condition). The form ofthe stent is one of a “ring and link” structure, in which shortcylindrical rings 502 are connected to each other by links 504 extendingparallel with the elongate axis of the stent. Each ring 502 is formed bya series of peaks 508 and valleys 510 that are connected to each otherby arms 506 to provide a ring that extends the circumference of acylinder while adopting an oscillating or wave-like shape. Peaks 508 onone ring are connected to valleys 510 of an adjacent ring to make up thestent, which has a great amount of flexibility in the longitudinaldirection.

Of significance in the present invention, however, is that the axiallength of each ring 502 in the compressed condition decreases from theproximal end towards the distal end. This is understood with referenceto FIG. 12, where there is shown a stent having “n” rings along itslength. The first ring on the proximal end has a length L1, the secondring has a length L2, and on . . . to the nth ring at the distal endwhich has a length Ln. As shown in FIG. 12, the length of L1 is longerthan L2, which is longer than L3, . . . all the way to Ln which has theshortest length. In a preferred embodiment, the rate at which the ringsshorten in length is constant, by which it is meant that the ratio ofthe length of any ring to the length of the preceding ring is a constantnumber. Thus, for example, if there are “n” rings, the length of thefirst ring is L1, and the ratio of L2/L1 is 0.95, then the length of the“nth” ring Ln will be L1*(0.95%)^(n−1) and, in a practical example, ifn=10, then the “nth” ring will have length L1*0.63, or stated otherwise,63% of L1, with the length of the intermediate rings being evenlydistributed between L1 and L1*0.63 in length.

The structure shown in FIG. 12 may be cut from a cylinder having aconstant diameter over its length and a constant thickness, according toknown means by laser energy. The number of peaks and valleys arepreferably the same in each ring 502, and the number of links 504 arethe same between each ring. The only aspect that varies from ring toring is the length of each ring. Preferably, the length of each link mayalso vary, in order to allow the spacing between the rings to remainconstant.

The advantage provided by the structure described above is that thestent 500 will, upon expansion by a balloon, be capable of adopting aconfiguration such as is shown in FIG. 13, in which each ring isexpanded to a similar degree, yet the stent will have a gradual taperfrom the proximal end narrowing to the distal end. The term “similardegree” to describe expansion of a ring indicates that when two ringsare expanded to a “similar degree,” then each arm 506 of each ring willbe angled at substantially the same angle to the elongate axis of thestent. For example, the angles shown as E, F, G in FIG. 13 are allsubstantially equal to each other, and reflect the fact that the ringsidentified are expanded to a similar degree to each other. Thisgeometric consequence follows necessarily from the fact that, accordingto the geometry of the rings, the proximal rings have a longer lengththan the distal rings. It will be appreciated by one of ordinary skillthat a stent in which all the rings are expanded to a “similar degree”will possess expanded rings that possess similar radial strength to eachother. This is a desirable outcome, and has an advantage over a stentwith identical rings that is deformed into a tapered shape, because insuch a stent the rings will be expanded to different degrees, and willpossess different radial strength from each other.

It will be appreciated that the stent described in reference to FIGS.12-13 will benefit from the balloon embodiments that have beendescribed. The balloon embodiments are capable of providing a balloonthat can be shaped with a relatively precise taper after the balloonpasses beyond nominal pressure. Further, the stent may be cut so that italso will expand with a relatively precise taper, while at the same timepossessing a uniform and constant radial strength along its length.

Thus, the balloons and stent of the present invention provide anadvantageous structure and method for improving the apposition of stentswithin tapered vessels. The present invention may, of course, be carriedout in other specific ways than those herein set forth without departingfrom the essential characteristics of the invention. The presentembodiments are, therefore, to be considered in all respects asillustrative and not restrictive, while the scope of the invention isset forth in the claims that follow.

We claim:
 1. A balloon for attachment to a distal portion of a medicalcatheter, the balloon comprising: a center portion having a proximalend, a distal end opposite the proximal end, and a length between theproximal end and the distal end, wherein the center portion furthercomprises: a first nominal diameter and a first radial modulus at theproximal end; a second nominal diameter and a second radial modulus atthe distal end; wherein, the first nominal diameter is equal to thesecond nominal diameter, such that, when the balloon is inflated to anominal pressure, the center portion has a constant diameter over thelength; and further wherein, the first radial modulus is smaller thanthe second radial modulus, such that, when the balloon is inflated abovea nominal pressure, the center portion adopts a tapered shape in whichthe proximal end has a first stretched diameter and the distal end has asecond stretched diameter, the first stretched diameter being largerthan the second stretched diameter.
 2. The balloon of claim 1, whereinthe center portion comprises a compliant polymer membrane which has afirst thickness at the proximal end and a second thickness at the distalend, wherein the first thickness is less than the second thickness. 3.The balloon of claim 1, wherein the center portion comprises a compliantpolymer membrane and wherein a plurality of successive threads arewrapped circumferentially around the center portion to reinforce thecenter portion, the threads being spaced along the center portion at aconstant pitch and being adhesively attached to the center portion;further wherein an initial successive thread is located at the proximalend and has a first cross sectional area, a final successive thread islocated at the distal end and has a second cross sectional area, and amedial successive thread is located between the initial successivethread wherein the final successive thread has a third cross sectionalarea, and further wherein the first cross sectional area is smaller thanthe second cross sectional area and the third cross sectional area islarger than the first cross sectional area but smaller than the secondcross sectional area.
 4. The balloon of claim 1, wherein the centerportion comprises a compliant polymer membrane and wherein an initialtwo consecutive threads are located at the proximal end and have a firstpitch between them, a final two consecutive threads are located at thedistal end and have a second pitch between them, and a medialconsecutive two threads are located between the initial two consecutivethreads and the final two consecutive threads and have a third pitchbetween them, wherein the first pitch is larger than the second pitchand the third pitch is smaller than the first pitch but smaller than thethird pitch.
 5. The balloon of claim 1, wherein the center portioncomprises a compliant polymer membrane and wherein a first thread iswound in a helix along the center portion and a first two successivewindings are located at the proximal end and have a first pitch, a finaltwo successive windings are located at the distal end and have a secondpitch, and a medial two successive windings are located between thefirst two successive windings and the final two successive windings andhave a third pitch, wherein the first pitch is larger than the secondpitch and the third pitch is smaller than the first pitch but largerthan the third pitch.
 6. The balloon of claim 1, wherein the centerportion comprises a compliant polymer membrane and wherein an initialsuccessive thread is located at the proximal end and is formed from amaterial having a first elastic modulus, a final successive thread islocated at the distal end and is formed from a material having a secondelastic modulus, and a medial successive thread is located between theinitial successive thread and final successive thread, and is formedfrom a material having a third elastic modulus; wherein the firstelastic modulus is smaller than the second elastic modulus and the thirdelastic modulus is larger than the first elastic modulus but smallerthan the second elastic modulus.
 7. A stent for insertion into a vesselof a patient comprising: a plurality of rings that are successivelyconnected to each other by a plurality of links, the plurality of ringsextending in an axial direction from a first ring at a proximal endfollowed by a plurality of succeeding rings to a final ring at a distalend, wherein, each succeeding ring is preceded by a preceding ring; eachof the plurality of rings includes a plurality of adjacent peaks andvalleys, wherein each valley is connected to an adjacent peak by a strutto provide an undulating pattern within each ring; and each of theplurality of rings has a compressed condition for delivery into thepatient and an expanded condition after deployment in the patient,wherein, in the compressed condition each preceding ring has a precedingring length measured in the axial direction and each succeeding ring hasa succeeding ring length measured in the axial direction, wherein aratio of each succeeding ring length divided by each preceding ringlength is a constant number that is smaller than unity.
 8. The stent ofclaim 7, wherein the first ring has a first ring length and is connectedto a second ring by a first link having a first link length, the firstlink length being equal to the first ring length.
 9. The stent of claim7, wherein the ratio is in a range of 0.90 to 0.95.
 10. A method ofexpanding a stent within a vasculature of a patient comprising:disposing a stent upon a balloon that is deflated, the ballooncomprising a center portion having a proximal end, a distal end oppositethe proximal end, and a length between the proximal end and the distalend; inserting the balloon inside the vasculature of the patient;inflating the balloon to a nominal pressure and, simultaneously,imparting a cylindrical shape to the center portion of the balloon; andfurther inflating the balloon to a pressure beyond nominal pressure and,simultaneously, imparting a tapered shape to the center portion of theballoon.
 11. The method of claim 10, wherein imparting a cylindricalshape to the center portion of the balloon includes imparting acylindrical shape to the stent.
 12. The method of claim 10, whereinimparting a tapered shape to the center portion of the balloon includesimparting a tapered shape to the stent.